1. Field of the Invention
The present invention concerns a niobium powder and an electrolytic capacitor formed by using the powder.
2. Description of the Related Art
While tantalum powder has been used so far for electrolytic capacitors, it involves a problem that the deposit is limited and its supply cost is not stable. In recent years, a trend of using niobium of rich deposit and available at reduced cost for the anode of electrolytic capacitors has been promoted. However, electrolytic capacitors using niobium have several drawbacks compared with those using tantalum. A primary problem is that oxide films of niobium have poor thermal stability. This results in leak current due to thermal loads upon mounting of parts to bring about deterioration of capacitors.
As a countermeasure, nitrogen addition has been known in order to decrease the leak current (for example, refer to Metal, vol. 72 (2002), No. 3, p 221). However, such a countermeasure can not be effective for improving the thermal stability.
A technique of reducing niobium pentachloride with hydrogen into a niobium powder has been present. However, since hydrogen remaining in the niobium powder after reduction reaches as much as 0.7 to 0.8 mass %, an electrolytic capacitor using the powder shows large leak current and large leak current after application of thermal loads, and is somewhat poor in static capacitance.
On the contrary, the present inventors have previously proposed a niobium powder containing small amount of hydrogen 1 to 600 ppm and a substantial balance of niobium (for example, in Japanese Patent Application No. 2002-22824 (pages 2-6)). The niobium powder described above is produced by applying a heat treatment, for example, at a temperature of about 1000xc2x0 C. or higher in an Ar atmosphere, to a niobium powder obtained by reducing niobium pentachloride with hydrogen described above. When this is used for an anode of a solid electrolytic capacitor, it provides excellent performance having large static capacitance of the capacitor and showing less leak current.
A solid electrolytic capacitor has a structure in which niobium, niobium oxide, solid electrolyte, graphite, silver, etc. are laminated and it is manufactured by sintering a niobium powder at 1000 to 1400xc2x0 C. to prepare a porous sintered product, applying a formation treatment to form a niobium oxide film on niobium, then forming solid electrolyte, graphite and silver successively into a layerous form and, finally, connecting an anode as an external terminal to niobium.
The present invention has been accomplished by further improving the invention in Japanese Patent Application No. 2002-11824 described above and the invention intends to provide a niobium powder for preparing a solid electrolytic capacitor with less leak current, less degradation of leak current after application of thermal loads and having high capacitance by enhancing stability of niobium oxide films in the capacitor, as well as a solid electrolytic capacitor using the powder described above.
The present invention provides a niobium powder comprising 0.005 to 0.10 mass % of hydrogen, 0.002 to 5 mass % of sulfur, in which a specific surface area of the powder is from 1 to 10 m2/g.
Further, the powder preferably comprises one or both of magnesium and aluminum by 0.002 to 1 mass % for further improvement of characteristics.
Further, an average particle diameter of secondary particles of the powder is preferably from 10 to 200 xcexcm.
A solid electrolytic capacitor having a sintered product from one of the niobium powders described above as an anode of it has excellent performance in view of static capacitance and leak current characteristics.